Dry plate rectifier



Patented Aug. 3, 1948 par PLATE RECTIFIER Stanley S. Fry, North Chicago,Ill., assignor to Fansteel Metallurgical Corporation, North Chicago,11]., a corporation of New York No Drawing. Application November 11,1944, Serial No. 563.108

11 Claims. I l

This invention relates to blocking layer devices such as seleniumrectifiers and light sensitive devices, and more specifically to meansfor improving the characteristics of selenium employed in v themanufacture of the blocking layer devices.

In the production of blocking layer devices having a semi-conductorlayer, for example, selenium rectifiers, a rigid carrier electrode orsupporting electrode is provided with a thin coating or layer ofselenium. The supporting electrode may be. formed of nickel,nickel-plated iron, aluminum, magnesium, berylium or other metals andalloys. A common practice in the production of selenium rectifiersincludes grit blasting the iron disc or plate and plating the disc withnickel. The purpose of the grit blasting is to present a roughenedsurface to the selenium and to thereby improve the adherence of theselenium to the carrier electrode. The disc is then thoroughly cleanedand a thin film of selenium is applied over the nickel layer.

The selenium film or layer may be formed by a variety. of methods. Theplate or disc may be heated t6 a temperature above the melting point ofselenium, for example, to a temperature of from 230 to 250 0., and theselenium in stick form ma be rubbed across the heated plate in ordertomelt the selenium and form the desired film. Another method ofapplication includes placing a, measured quantity of powdered seleniumor selenium in pellet form on the heated plate and flowing the meltedselenium over the surface. The melted selenium is usually spread overthe heated plate by mechanical means, as with a glass rod. The seleniummay also be deposited on the carrier electrode from a vapor phase. Thevaporization method is commonly employed in depositing the selenium filmupon the light metal carrier electrodes.

The selenium is then transformed into its gray, crystalline state byheat treatment. The coated discs are generally stacked with mica,aluminum or other inert, smooth-surfaced discs or washers in contactwith the selenium and between adjacent plates, and the stack subjectedto a moderate pressure. The stacks under pressure are then subjected toa relatively low temperature, that is, a temperature below 150 C., andmaintained at this temperature for from one hour to four or five hours.During this stage of the heat treatment the selenium softens so that thepressure produces a layer of selenium of relatively uniform thicknessand having a smooth surface. I

The stacks are withdrawn from the oven or heat treating furnace and theplates are removed 2 from the stack. The plates are then given a furtherheat treatment at a temperature approaching the melting point ofselenium. This heat treatment is generally conducted at temperaturesbetween about 200 C. and the melting point of selenium, for example,about 210 C., for a period of from fifteen minutes to several hours.During the combined heat treatment, a layer of more or less uniformthickness of selenium is produced having a. smooth surface and theselenium is transformed from its amorphous, non-conducting form into itsgray, crystalline, conducting form.

The smooth surface of the selenium film is then treated to form ablocking layer and a counterelectrode consisting of a relatively lowmelting point alloy is applied, as by spraying, over the seleniumcoating.

The final step in the manufacture of the rectifier plates consists of anelectrical forming treatment. This treatment consists of subjecting theplate to either an alternating or direct current until a high resistanceis developed in the reverse direction. This step may consist of applyingto the plate in the reverse direction a direct current voltage of belowabout 15 volts or a ward direction of the rectifier plate. The voltagemay be about 20 volts and the period required for forming is generallygreater than that required when a direct current is employed becauseonly one-half cycle of the current flows in the reverse direction. I

Light sensitive devices of the blocking layer class are produced in asimilar manner. A supporting electrode is provided with a thin film orlayer of selenium in its gray, crystalline modification and anartificial blocking layer formed on the surface of the selenium. Asecond electrode is then applied over the treated surface of theselenium. This electrode may consist of a light permeable film of metalor a metal grid. The application of the light-permeable metal may beaccomplished by any of the methods known to the art.

The present invention is directed to means for improving theconductivity of the selenium layer in the forward direction. Theinvention is not limited to any specific form, type or composition knownmethods.

of carrier electrode or counterelectrode. Any desired method may beemployed in forming an artificial blocking layer on the seleniumsurface. In the preparation of selenium rectifier plates any means forelectrically forming the rectifier plates may be employed.

The principal object of this invention is to improve the forwardconductivity of the selenium layer in blocking layer devices.

A further object of this invention is to increase the ratio of theresistance in the reverse direction as compared to the resistance in theforward direction.

Other objects and advantages of this invention will become apparent fromthe following description and claims.

The present invention contemplates incorporating in the selenium film orlayer a small amount of methylene iodide The specific details of themanufacturing process are dependent upon the type of unit being made andthe class of service for which it is desired. For purposes ofillustration the production of selenium rectifier plates is described.

The methylene iodide is added to molten selenium in the proportions offrom about 0.05 cc. to about 0.5 00., preferably about 0.1 cc., to 100grams of selenium. The selenium is melted and agitated vigorously whilethe methylene iodide is added. The mass is then cast promptly into thedesired form.

The carrier electrode. for example, a nickelplated iron disc is providedwith a layer or film of the doped selenium in its gray, crystallineform. This layer may be formed by heating the carrier electrode andapplying the doped selenium in powdered form or by rubbing a stick ofdoped selenium over the heated carrier electrode. The resulting film orlayer is then transformed into its gray, crystalline form by the usualheat treatment. The selenium surface is then treated to form anartificial blocking layer by any of the The counter-electrode is thenapplied by spraying a low melting point alloy over the treated seleniumsurface. The rectifier plate is completed by subjecting the compositeunit to an electrical forming process.

The addition of these small amounts of methylene iodide to the seleniumdecreases appreciably the forward resistance of the selenium anddecreases the ratio between the current flow in the reverse directionand the current flow in the forward direction upon the application of adirect current voltage to the plate in the reverse and forwarddirections. An ideal or theoretically perfect rectifier plate would haveno current flow in the reverse direction and the rectification ratiowould be zero.

In order to compare the rectification ratio of rectifier plates formedwith selenium and rectifier plates formed with selenium containing smallamounts of methylene iodide, a sample of sele-' nium was divided intotwo portions. One portion was heated to about 350 C. and 0.1 cc.methylene iodide added per 100 grams of selenium. The mass was agitatedvigorously durin the addition and was then cast in stick form. The otherporsticks over the heated plates and spreading the melted material overthe surface with a glass rod. The plates were then stacked with micadiscs covering the selenium layer and the stacks placed under pressure.The stacks were heated to about 140 C. and maintained at thistemperature for about 2 hours after which the pressure was retion ofselenium was melted and cast in stick moved and the plates heated toabout 210 C. and maintained at this temperature for about 5 2 hour.

The plates were subsequently provided with a counterelectrode and thensubjected to the same electrical forming operation. The plates formedfrom untreated selenium were intended as standard or control plates.

A rectified direct current voltage of 1 voltwas applied to theindividual plates of each series in the reverse and in the forwarddirections and the current flow measured. The leakage current or reversecurrent flow in the standard or control plates averaged approximately0.35 milliampere and the forward current flow in these plates averagedapproximately 0.42 ampere. The rectification ratio of these plates was1: 1200.

The leakage current or reverse current flow in the case of the platesmade in accordance with the present invention averaged approximately0.40 milliampere. The forward current flow in these plates averagedapproximately 0.83 ampere. The rectification factor of such plates was1:2075.

The current flow through the individual plates of both series was alsomeasured at an applied rectified direct current voltage of 3 volts. Theleakage current flow in the case of the standard or control platesaveraged approximately 1.2 milliamperes and the forward current flowaveraged approximately 2.5 amperes. The rectification ratio at 3 voltsfor these plates was 1:2083,

The leakage current or reverse current flow in the case of the platesmade from the doped selenium averaged approximately 1.35 milliamperesand the forward current flow averaged approximately 4.4 amperes. Therectification ratio for this series of plates at 3 volts was 1 3260.

The foregoing numerical values of current ilow 'will differ withdifferent plate areas and also with diiferent grades and samplesofselenium. These values, however, are representative of thecharacteristics imparted by the addition of methylene iodide to seleniumand the improvements realized by the use of the doped selenium areapparent from this data.

I claim:

1. The method of producing blocking layer de-- vices which comprisesadding a small amount of methylene iodide to selenium, providing asupporting electrode wtih a layer of gray, crystalline seleniumcontaining the methylene iodide, forming a blocking layer on the surfaceof the selenium layer and applying a counter-electrode over the treatedselenium surface.

. 2. The method of producing blocking layer de-- vices which comprisesadding a small amount of methylene iodide to selenium, applying a layerof the selenium containing the methylene iodide to a supportingelectrode, transforming the selenium into its gray, crystalline form,forming a blocking layer on the selenium surface and applying acounter-electrodeover the treated selenium surface.

3. The method of producing blocking layer devices which comprises addingmethylene iodide to selenium in the relative proportions of from about0.05 cc. to about 0.5 cc. methylene iodide per gms. of selenium,applying a layer of the selenium containing the methylene iodide to asupporting electrode, transforming the selenium into its gray,crystalline form, forming a blocking layer on the selenium surface andapplying a counterelectrode over the treated selenium surface.

a 4. The method of producing selenium rectifier plates which comprisesadding a small amount of methylene iodide to selenium, providing a supporting electrode with a layer of gray, crystalline selenium containingthe methylene iodide, forming a blocking layer on the surface of theselenium layer, applying a counter-electrode over the treated seleniumsunface and subjecting the composite unit toan electrical formingoperation.

5. The method of producing selenium rectifier plates which comprisesadding to selenium a small amount of methylene iodide, app ying a layerof the selenium containing the methylene iodide to a supportingelectrode, transforming the selenium into its gray, crystalline form,forming a blocking layer on the selenium surface, applying acounter-electrode over the treated selenium surface and subjecting thecomposite unit to an electrical forming operation.

6. The method of improving the conductivity of selenium which comprisesincorporating in selenium a small amount of methylene iodide.-

7. The method of improving the conductivity of selenium which comprisesincorporating in selenium from about 0.05 cc. to about 0.5 cc. methyleneiodide per 100 gms. selenium.

8. The method of improving the conductivity of selenium which comprisesmelting selenium and incorporating in the molten selenium a small amountof methylene iodide.

9. A blocking layer device comprising a carrier electrode, a layer ofselenium to which has been added a small amount of metirvleneiodide anda counter-electrode.

10. A blocking layer device comprising a carrier electrode, acounter-electrode and an intermediate'layer of selenium .to which hasbeen added from about 0.05 cc. to about 0.5 cc. methylene iodide per l00gms. of selenium.

11. A selenium rectifier comprising a carrier electrode, acounter-electrode and an intermediate layer of gray, crystallineselenium to which has been added a small amount of methylene Y iodide.

STANLEY B. FRY.

REFERENCES CITED The following references are of record in the file ofthis patent:

. UNITED STATES PATENTS Handbook of Chemistry and Physics, 26th edition,1942-1943; Chemical Rubber Publishing 00., pages 880-881.

